The Forsyte family gather to celebrate the engagement of Winifred Forsyte to Montague Dartie, but Winifred's parents, Emily and James, disapprove. Soames Forsyte, a rich and successful partner in the family law firm, meets Irene Heron whilst in Bournemouth on business. She's penniless, but beautiful and accomplished and agrees to marry Soames after an aggressive courtship.

Soames is concerned about his marriage and hires Bosinney to build a country house at Robin Hill for Irene, bringing the couple together and they soon fall in love. After years apart, old Jolyon gets in touch with his son young Jolyon, who now has two children, Jolly and Holly. Dartie and Winifred's marriage is a success, but Dartie's love of fast living and gambling causes money problems.

Irene and Bosinney plan to run away together and Soames is furious that the project is overspending again and decides to sue Bosinney. But a tragic series of events - including rape - ensues from the night before the hearing which ends with an horrific death. But Irene's troubles are far from over as Young Jolyon makes a decision that will haunt him.

Grieving after Bosinney's death, Irene runs away. Penniless she lives on the streets but is rescued by another destitute woman. Soames is forced to come to terms with losing her. 'Old' Jolyon reunites his family and they move to Robin Hill. Helene dies shortly after. The years pass. 'Young' Jolyon, June and Jolly take a trip to Europe, leaving 'Old' Jolyon with Holly now 11. Summertime - Old Jolyon sees Irene in a secluded part of the garden. She becomes a regular guest and they become great friends. 'Old' Jolyon dies one sunny afternoon, leaving £15,000 in his will to Irene, which incenses the Forsyte family.

1899 - bankrupt after gambling losses Dartie steals Winifred's pearls, and threatens to shoot her when confronted. He leaves. She starts divorce proceedings. Soames is lonely and bitter about the past, and wants an heir. Frenchwoman Annette would make him a suitable wife, but first he must divorce Irene. He visits 'Young' Jolyon, taking his nephew Val Dartie with him. Val and Holly are instantly attracted. Soames visits Irene and tries to force her to return. Terrified she flees to Paris, Jolyon visits and they fall in love. Soames hires a private investigator to follow Irene. Jolly and Val enlist to go and fight in the Boer war, meanwhile Dartie returns.

Soames goes to Paris for one last chance with Irene. She tells him she hates him. Irene and Jolyon become lovers and are named in the divorce petition. They want to marry, and soon she is pregnant and has a son, Jon. Soames marries Annette; his father James is desperate for a grandson before he dies.